Viewed in X-rays the night sky glows brightly and uniformly in all directions.
The origin of this 'cosmic X-ray background' was one of two great puzzles set
by the very first observations of X-rays from beyond the Solar System by
Riccardo Giacconi and his team back in 1963. (For which Giacconi won the Nobel
Prize in Physics in 2002 (see:Riccardo Giacconi .) To solve the puzzle required sensitive X-ray telescopes, culminating in the Chandra X-ray Observatory . This X-ray glow is due to the summed emission
from millions of faint quasars and active galactic nuclei that can now be seen
individually in deep Chandra images.
To discover this answer required surveys of the sky away from the confusing
and absorbing plane of the Milky Way, so objects in our Galaxy contribute
little
leading these to be called 'Extragalactic Surveys'. Even though we no know the
answer, HEA scientists continue to make new extragalactic surveys to learn
more about how quasars evolve over the history of the universe, and to study
even fainter sources - distant galaxies and, eventually, the first stars,
galaxies and black holes to form in the universe.